Solar System:Satellites & Summary
Melissa A. McGrathSpace Telescope Science Institute
n Determine the evolutionary processes that led to the diversity of Solar System bodies and the uniqueness of Earth
n Use the other objects of our Solar System as natural science laboratories
Broad Goals (COMPLEX, NASA strategic plan)
Solar System science is different because s/c exploration makes many of our targets more observationally mature.
[We are doing a lot more “weather” than the rest of you…]
Io
Europa
Ice rafting
Karkoschka 1998
Satellites science with HST: Greatest Hits
Leading Anti-Saturn
Saturn facing Trailing
Smith et al. 1996
Detection of tenuous oxygen atmospheres on Europa & Ganymede
1356/1304 ratio ~ 1-2à O2 gas
Hall et al. 1994Hall et al. 1998
GHRS spectroscopy
Aurora on Ganymede confirmed by HST
STIS imaging spectroscopyFeldman et al. 2000
OI] 1356A emission
Galileo discovery of a magnetic fieldon Ganymede
Gurnett et al. 1996Kivelson et al. 1996
Detection of solid state absorbers on many icy satellites
n SO2 in ice on Europa and Calliston O3 in ice on Ganymede, Rhea, Dionen O2 (solid state) on Ganymede
FOS SpectroscopyNoll et al. 1995, 1996, 1997;Calvin and Spencer 1997
Saturn Ring Plane crossing
Triton stellar occultationFGS scan – Elliot et al. 1998
Model-derived T and P
Global warming on Triton
Pluto – methane ice
Charon – water ice
Figure courtesy M. Buie, W. GrundyLowell Observatory
Io: The most observed satellite
Given its small size, andlocation deep within thegravitational well andmagnetic cavity, it has a huge impact on the Jovian system…
Io & the Jovian magnetosphere
Diagram courtesy John Spencer, Lowell Observatory
Io, 6Rj1028 S,O/sec
Io plasma torus
Relative motion of plasma & satellite:
induces corotational E fieldEi = -vrel x B
57km/sec x 2000nT ~500kV potential across Io drives currents of few x 106 Amps
Satellite signatures in the Jovian aurora
HST/STIS image courtesy John Clarke
Io
EuropaGanymede
Sodium CloudMendillo et al.
Meaningful studies of the Io volcanoes from Earth vicinity
HST/WFPC2 - Pele
Spencer et al. 1997
Detection of SO2, SOPlume spectroscopy
FOS 0.3” aperture Pele volcano
McGrath et al. 2000
atomic sulfur emission
Plume spectroscopy
STIS long-slitspectroscopy ofPele plume
Specner et al. 2000
Detection of S2 in the Pele plume
HI Lyman-αα images (1215.67A) of Io
Dark = more SO2 gas
1998 observationsFeldman et al. 2000
A “picture” of theSO2 atmosphere
1999 observationsMcGrath et al. 2001
Ø Surface T is not axisymmetric, it’s colder at poles
Ø Atmosphere is not global, it falls off w/ latitude
Ø Atmosphere is obviously variable
Motion of “spots” changes with B field orientation
OI] 1356 emissionRoesler et al. 1999
“Aurora” on Io
n Cassini at Saturn: 2004-2008 (nominal)
n Outer planets mission priorities beyond Cassini:« Pluto: earliest possible arrival 2015, KBO
arrival ~2025« Europa orbiter: date unclear at this time
Upcoming missions of relevance for satellites
n Galilean satellites will continue to be primary targets – we are still (almost always) photon starved in the UV
Satellite science w/ next UV/opt telescope(s)
n Enhanced capabilities will:« Allow detailed follow-up to Cassini w/ “Io-like”
science for Titan and mid-size Saturn satellites« Open up the distant solar system (Uranus, Neptune,
Pluto/Charon, etc.), which remains largely unexploredn In future there will be a bigger focus on nitrogen
(N2, NH3) in the outer solar system (Titan, Triton, Pluto, Charon atmospheres and surfaces). Cryovolcanism may be important on more distant icy satellites (and there are lots of them!).
n Another Saturn ring-plane crossing in 2010, then 2024
Possible for Triton (& others?)
Spencer et al. 1997
n Serendipity – SL9 was one of the greatest events of all time w/ HST
n Synergy with other NASA missions(HST-Galileo; HST-Cassini; HST-Lunar Prospector; HST-Chandra; HST-MGS)
n It sells well in Peoria (=Capitol Hill)Planetary science is very popular, and is PR’d disproportionate to the actual amount of observing time (~5% per cycle with HST)
Other compelling reasons to support solar system science w/ NHST
n Ground-state/resonance transitions of many atoms, ions, and molecules. « H2, H, O, C, S, N, SO2, S2, N2, CO, CO2, …« Io and its plasma torus have perhaps the richest S,O
emission line spectra known
n Many UV absorbers important for planetary atmospheres: hydrocarbons (CH4, C2H2, C2H4, etc.); NH3; SO2, SO, S2, …
n Their solar type spectrum, combined with very low UV reflectivities combine to give very UV continuum compared to the visible, making energetic emission line processes such as aurorae and dayglow detectable.« Jovian aurora not detectable in (visible) Hα from Earth
Why the UV?
n First: don’t preclude planetary observations in the early stages. (E.g., level 2 requirements do not preclude moving target tracking for NGST.)
Summary & Discussion lead-in
Then: at minimum do the simple things that enable planetary observations. « Example: many missions (IUE, HST, HUT) have solar
avoidance limit that (barely) precludes Venus. Is this technically necessary, or “historical”?
n Moving target tracking « Include requirement for capability from the beginning or it
probably won’t happen.« It can be relatively simple and low-cost.
n More “modes” is not always the answer« Example: planetary slits on STIS (not being used)
n FOV/spatial resolution« High resolution imaging channel large enough for
Jupiter (=large enough for Saturn+rings), ~50”
HST/WFPC2 HST/STIS
« As many resolution elements as possible for small targets: Pluto (<0.09”), Triton (<0.13”)Fiducial: 8m @ 0.25µm = 15 (λ/D)s across Pluto
Stern et al. 1997
Pluto/Charon FOC imaging:Pluto ~7.5 pxs diameter (~3 λ/D),0.92” separation
Albrect et al. 1994
n FOV/spatial resolution
« Large FOV (degrees) desirable for KBOs and comets. But, ~400 KBOs now known, many more expected by 2010 from g-b searches. Like HST & NGST, HST IIs niche will be the small, faint end of the distribution, which does not require large FOV.
n For almost all the science we’ve shown today we are still photon starved in the UV. The very factors that make the UV emissions detectable also makes them hard to detect…
n How much more? As much as we can possibly get, both via the aperture and the detector technology improvements.
n For example, the UV emission N2/N spectrum of Titan has not been detected (at all) since 1980 by the Voyager UVS, and it too should have an interesting interaction with the Saturn magnetosphere
Sensitivity
n Solar blind detectors are criticaln Lyα is important (à above geocorona)n UV below Lyα is importantn Near-UV is important
Spectral coverage
Spectral resolution/Slitsn R = 10,000 adequate for most problems; 30,000
would give Doppler shifts and winds on, e.g., Ion Slits: long & narrow like STIS
« Long for throughput and spatial information« Narrow for spectral resolution on large, extended targets
[slit width = resolution element]
n Slitless spectroscopy useful (e.g., STIS on Galilean satellites)
HST/STIS imaging spectroscopy
McGrath et al. 2001
geocoronal emission
Europa
spat
ial
Distribution of dense-phase (sub-surface) O2 on Ganymede
FOS spectroscopyCalvin and Spencer 1997
n Time-tag/rapid read out (~0.1s resolution) is important (e.g., occultations)
n Low Earth Orbit gives no CVZ for solar system targets
n Many problems could be addressed w/ relatively small (HST class) aperture with continuous time coverage (planetary weather/climate, aurorae, satellite-magnetosphere interactions)
n Smaller solar avoidance to allow Venus, comets in inner solar system (HST limit is 50o)
Temporal coverage (synoptic monitoring)
n Cassini at Saturn: 2004-2008 (nominal)n Outer planets mission priorities beyond Cassini:
« Pluto: earliest possible arrival 2015« Europa orbiter: date unclear at this
n Upcoming NASA & ESA« On-going Mars at most 24-month opportunities« Contour & Rosetta (comets)« Deep Impact (comet penetrator)« Messenger & Beppo-Colombo (Mercury)« DAWN (asteroids)
n New Frontiers line ($600M cap) in President’s 2003 budget
Upcoming missions of relevance for Solar System
n H2 and H emission (aurorae)n Desire to get above geocoronan Desire to go below Lyαn Imaging spectroscopyn Synoptic monitorinn 1 mas in the UVn “weather” ☺
There is a lot of commonality with needs/interests already discussed
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